1. Field of the Invention
The present invention is broadly concerned with an improved aqueous sympathetic ink composition particularly adapted for use with any non-impact printers using liquid inks, e.g., ink jet printers. More particularly, the invention pertains to such a composition, and a corresponding method of use, wherein the ink composition preferably includes therein a water soluble transition metal salt and an amount of n-butanol. The ink of the present invention is designed to dry as an essentially invisible image on a substrate, which can later be developed by application of a color precursor developer.
2. Description of the Prior Art
Modern day sympathetic ink systems of commercial importance are designed for a variety of uses, for example in self-testing educational booklets. In such uses, a student may be provided with a multiple choice answer sheet, with the correct answer being indicated by appropriate latent image indicia. In selecting an answer, the student marks one of the multiple choice possibilities with the developer substance; if his selection is correct, the latent indicia will reveal the student's correct choice and hence his progress. Other uses include certification testing, personnel selection and screening tests, novelty items such as games and toys, games of chance such as lotteries, children's books designed for educational and/or entertainment purposes, securing of documents, prize verification and promotional items.
Such sympathetic ink systems typically involve application of a latent image by means of a normally invisible printing ink. Thereafter, a developer substance is applied over the invisible image and a color-forming chemical reaction occurs to "develop" the image and render the same visible.
U.S. Pat. Nos. 4,631,203 and 4,784,876 describe particularly useful sympathetic ink/developer systems which give sharp, long-lasting, non-smearing images. However, these systems are designed for use with conventional lithographic, letter press, letterset, flexographic, silk screen or rotogravure printing equipment and hence the ink systems thereof are typically rather thick and viscous.
In many applications, however, it would be advantageous to employ non-impact printers such as ink jet printing equipment in the production of latent imaged items. Typical sympathetic ink systems of the type described in the aforementioned patents are not suitable for use in ink jet printers, because they have inappropriate viscosity and surface tension properties. As a consequence, it has heretofore been impossible to employ ink jet printers to give latent imaged products, even though the flexibility and printing characteristics inherent in such equipment would otherwise make it ideally suited for this application.